As a first step toward the long term goal of understanding species differentiation at the molecular level, we plan to study the recently cloned gene of reproductive isolation in Drosophila, Odysseus (Ods). Introgression of an appropriate D. mauritiana chromosomal region containing the Ods locus causes male sterility in D. simulans (Perez et al. 1993; Perez and Wu 1995). Recent progress suggests that Ods is a homeodomain-containing gene. This gene is unusual in that its homeodomain has experienced extremely rapid amino acid substitution in Drosophila. Its rapid evolution coupled with the newly acquired expression in male germline is manifested as hybrid male sterility. The objectives of this proposal are: 1) To investigate the molecular basis of Ods divergence by germline transformation: Both the causes of hybrid sterility and the normal functions of Ods will be explored by exon swapping in transgenic studies. 2) To study the expression pattern of Ods by in situ antibody staining: The correlation between the expression in male germ cells, the rapid sequence evolution and the genetic incompatibility leading to sterility will be explored. 3) To analyse the DNA binding property of the Ods homeodomain protein: This will be a first step toward the eventual identification of downstream genes. 4) To study the molecular evolution and population genetics of Ods: This will be the first DNA sequence analysis of a gene that plays a key role in reproductive isolation.